A nonwoven fabric for a battery separator has a function for separating an anode from a cathode, preventing electrode active material or electrode debris from migrating from one electrode to another to cause a short-circuit and for retaining an electrolyte therein. Also it must have a low internal resistance and/or a favorable chemical stability and durability against the electrolyte. Particularly, in a secondary battery such as a Ni-Cd storage battery, it is necessary that an oxygen gas generated upon overcharge due to a cathode reaction is smoothly consumed in an anode, and the electrolyte is prevented from being released from the separator due to the change in electrode thickness upon charging/discharging. A high gas-permeability and a high electrolyte holding capability are also required as a separator even after the compression is repeated. Also, a mechanical strength durable against a tension applied to the separator when the same is incorporated in a battery is one of the important requisites.
However, battery separators made of a conventional nonwoven fabric, such as a melt-blown type, a flash-spun type, a dry-laid type or a wet-laid type, have been unsatisfactory in the above requirement.
For example, Japanese Unexamined Patent Publication (Kokai) No. 1-157055 proposes the improvement in mechanical strength, electrolyte retention capability and short-circuit preventing ability, by heating a melt-blown type nonwoven fabric under a predetermined condition to form a compact structure on the surface thereof. However, the mechanical strength of the nonwoven fabric could not be improved by merely hot-pressing the same, but higher temperature and pressure are necessary for obtaining a desired mechanical strength, which results in a problem in that the electrolyte holding capability and gas-permeability deteriorate thereby. Japanese Unexamined Patent Publication (Kokai) No. 2-259189 discloses the improvement in mechanical strength and electrolyte retention capability of a nonwoven fabric by thermally bonding alkali-resistant fibers with hot-melt fibers in a wet-laid mixture fiber sheet. However, it is necessary to increase an amount of hot-melt fibers and increase the bonding temperature to obtain a higher mechanical strength, which results in a reduction in the number of voids in a fiber surface and a sheet to cause the deterioration of electrolyte retention capability. On the contrary, if a lower thermal bonding temperature is used, the mechanical strength becomes unsatisfactory for a battery separator and causes a problem in that the nonwoven fabric structure is liable to collapse upon compression, due to a poor interfiber entanglement, which results in the exhaustion of electrolyte in the separator; a so-called "dry-out"; and increases the electric resistance of the battery.
Japanese Unexamined Patent Publication (Kokai) No. 61-281454 discloses that a melt-blown type nonwoven fabric comprising fibers having a single fiber diameter in a range from 0.1 to 2 .mu.m is bonded with a sheet comprising fibers having a single fiber diameter of 5 .mu.m or more by highly pressurized water jets to result in a product suitably used for a battery separator, improved in air permeability and cathode/anode separation ability and capable of preventing short-circuit from occurring due to the migration of active material or debris from electrodes. However, the resultant nonwoven product has a plurality of through-holes on the surface thereof due to the highly pressurized water jets, which deteriorates the ability for preventing the active material from migrating. In Japanese Unexamined Patent Publication (Kokai) No. 5-89867, a method is disclosed wherein a nonwoven fabric is compressed with rolls having different peripheral speeds to contract the through-holes, but the through-holes, once formed on the surface of the nonwoven fabric, are difficult to eliminate.
Since the nonwoven fabric disclosed in the above-mentioned Japanese Kokai No. 61-281454 is a sheet of a double-layer structure comprised of a melt-blown type nonwoven fabric of ultra-fine fibers and a dry-laid web, surface areas and densities of fibers composing the layers are different between the respective layers in the cross-sectional direction and thus the electrolyte held therein becomes uneven to increase the internal battery resistance. Also, since the nonwoven fabric layer of melt-blown fibers lacks the uniformity in basis weight due to the spinning method itself, it is impossible to inhibit the migration of active material through a portion having a lower basis weight, whereby the short-circuit preventing ability becomes inferior. In addition, since the nonwoven fabric is liable to wear due to the compression thereof by the repetition of volumetric change caused by the charging/discharging of the secondary battery, the electrolyte may be released therefrom to deteriorate the durability of the secondary battery against the repetition of charging/discharging cycles.
In Japanese Unexamined Patent Publication (Kokai) No. 5-74440, there is a disclosure that, after a wet-laid mixture sheet of staple fibers, hot-melt fibers and a synthetic pulp is subjected to a fluid treatment, the hot-melt fibers and the synthetic pulp fibers are heat-bonded to each other to obtain a nonwoven fabric improved in short-circuit preventing ability, electrolyte retention capability and mechanical strength. This proposal, however, has a drawback in that the synthetic pulp fiber is not capable of maintaining its fibrous shape during the heat treatment for bonding the hot-melt fibers to reduce the surface area of the fibers in the nonwoven fabric structure and increase the interfiber distance, whereby the electrolyte in the separator easily migrates to electrodes to result in a rise in electrical resistance.
Japanese Unexamined Patent Publication (Kokai) No. 7-272709 discloses a battery separator having an improved short-circuit preventing ability by bonding fibrous material having a fibril-like bifurcate structure, such as synthetic pulp fibers, to a permeable interstitial structure of an entangled fibrous web obtained by subjecting a wet-laid web of splittable fibers to a hydroentangling treatment. However, since this separator is of a double-layered structure consisting of a surface layer of the entangled fiber web and a fibrous body having a fiber composition and structure different from the former, which is overlaid thereon in an irregular manner in accordance with the density variance of the entangled fiber web, the electrolyte is liable to be irregularly contained therein to increase the electric resistance or increase the variance of air permeability and electrolyte retention capability.
In addition, since the fibrous structure of this separator has a lower entangling density in its own structure as well as between the same and the entangled fiber web, the fibers in the fibrous body are easily released. Also, if the fibrous body is bonded to the entangled fiber web by the heat treatment, the fibrous shape disappears and, in an extreme case, the web becomes film-like, resulting in a problem in that the electrolyte retention capability deteriorates and the electrolyte is unevenly held in the surface layer.
In Japanese Unexamined Patent Publication (Kokai) No. 6-295715, there is a description in that a wet-laid sheet of acrylic staple fibers with added metallic ions, having a single fiber diameter in a range from 3 to 10 .mu.m and a ratio of fiber length L to fiber diameter D (L/D) in a range from 1000 to 2000 is hot-pressed, after being subjected to a water jet interlacing treatment, to result in a battery separator excellent in resistance to acids. However, this wet-laid nonwoven material is unstable because the three-dimensionally entangled structure is not fixed by fibers such as hot-melt fibers, whereby there may be a problem in that a widthwise contraction occurs due to a tension during the assembly of the battery, and exhaustion of electrolyte is generated due to the compression of separator upon the swelling of the electrode plate. Also, since the hydroentangling treatment is carried out without a rocking motion of a nozzle header, many continuous stripe marks in a warp direction appear on the surface of the nonwoven material, and this deteriorates the uniformity of formation thereof. Thus the short-circuit preventing ability in the irregular mark portion becomes worse when the sheet is used as a battery separator because the electrode active materials or others migrate therethrough.